The implementation of stricter emission limits for hydrogen sulfide (H2S) and sulfur dioxide (SO2) has stimulated the development and improvement of processes for the desulfurization of natural gas, synthesis gas, gasification streams and other gas streams used or generated in petroleum processing, oil recovery and coal utilization. For example, the level of H2S in natural gas must be lowered to 4 ppmv to meet pipeline specifications. Sulfur removal or desulfurization processes can also be applied to offgas generated in digesters or in waste water treatment, or to geothermal gases
Desulfurization is often required for applications other than natural gas, including purification of gasification streams, associated gas from wells, and various gas streams generated in petroleum refining.
Hydrogen and CO are the products of the gasification of coal, hydrocarbons, biomass, solid waste and other feedstocks. Gasification is most generally any process where carbon-containing materials are converted into product gases containing primarily carbon monoxide (CO) and hydrogen (H2). Various gasification processes are known and practiced in the art.
The product gas generated by gasification can be used to generate electricity or steam or can be used in chemical synthesis to make methyl alcohol (methanol), higher alcohols, aldehydes, or synthetic fuels (via Fischer Tropsch catalysis). Because one of the uses of gasifier product gas is to make chemicals, it is frequently referred to as synthesis gas or syngas (Satterfield, C. N. (1991) Heterogeneous Catalysis in Industrial Practice, 2nd ed., McGraw-Hill.) In most gasification processes, sulfur compounds present in the feedstock are converted into hydrogen sulfide, which appears in the product gas. Hydrogen sulfide must be removed from the CO and H2 mixture before the gas can be used for power generation because burning it generates sulfur dioxide emissions from the power plant. Hydrogen sulfide must be removed from the CO and H2 used for chemical synthesis because H2S irreversibly damages the catalysts used to make alcohols, aldehydes, and other products.
Mercury can be a significant contaminant in gas streams derived from fossil fuels such as coal (e.g., by gasification) and in natural gas. Mercury can also be a significant contaminant in flue gas emitted from coal-fired power plants. Mercury is removed from such gas streams to minimize its undesirable environmental release and to minimize the potentially detrimental effects that it may have on gas stream processing equipment. The levels of mercury present in any gas stream can vary widely dependent upon the type or gas stream and its origin.
Various methods have been reported in the art for the removal of mercury from gas streams, including injection or spraying of dry sorbents (e.g., activated carbon sorbents) or adsorbents, chemisorption methods (e.g., using chemically promoted sorbents), and scrubbing (e.g., with aqueous carbonate, sulfide or polysulfide solutions). The following U.S. patents or published PCT applications relate generally to removal of mercury from natural gas or synthesis gas (U.S. Pat. Nos. 5,034,203; and 5,053,209; and WO 94/22563) or to removal of mercury from combustion gases or flue gas (U.S. Pat. Nos. 6,214,304; 5,607,496; and 6,521,021). U.S. Pat. No. 6,589,318 relates to an adsorption powder for removing mercury from high temperature, high moisture gas streams.
Granite, E. J; Pennline, H. W; and Hargis, R. A. (April 2000) Industrial Engineering Chem. Res. 39:1020–1029 report the results of screening for sorbents capable of removing elemental mercury from various carrier gases. Sorbents screened included, among others, activated carbons, promoted activated carbons, certain metal oxides dispersed on supports (e.g., vanadium pentoxide and molybdenum oxide dispersed on a magnesium silicate support, manganese oxide or chromium-oxide supported on alumina and a ferric oxide sorbent); a platinum sorbent; a thiol promoted aluminosilicate sorbent and a molydenum sulfide sorbent. The reference also provides a brief overview of sorbent technology. U.S. Pat. No. 6,610,263 relates to methods and systems for removing certain pollutants from gas streams, such as combustion gases which employ sorbents or combinations of sorbents. Mercury is reported to be removed using an alumina sorbent. U.S. Pat. No. 5,827,352 relates to use of sorbent and water injection to remove mercury from gas streams.
U.S. Pat. No. 5,900,042 relates to a method to remove elemental mercury from a gas stream by first reacting the gas stream with an oxidizing solution to convert the elemental mercury to soluble mercury compounds and then passing the gas stream through a wet scrubber to remove the mercuric compounds and any oxidized constituents.
U.S. Pat. No. 6,248,217 relates to preventing release of heavy metals, such as mercury, using ultraviolet radiation to oxidize mercury, in exhaust gas, to its ionic forms which are less volatile. U.S. Pat. No. 6,576,092 also relates to removing elemental mercury from gas streams using UV irradiation.
U.S. Pat. No. 4,044,098 relates to a process for removal of mercury from gas streams using hydrogen sulfide and amines. The method is particularly applied to sulfur-free natural gas containing mercury. Hydrogen sulfide is added to the gas stream in excess of the stoichiometric amount needed to precipitate sulfides of mercury. Mercury is reported to precipitate out of the gas and the gas is then contacted with an amine to absorb hydrogen sulfide.
U.S. patent application Ser. No. 10/358,404, filed Feb. 3, 2003 and published as U.S. publication no. 2003 0194366A1 relates to catalysts and catalytic methods for the selective oxidation of hydrogen sulfide in a gas stream to generate elemental sulfur, sulfur dioxide or both. Employing the catalysts and methods therein, hydrogen sulfide can be selectively oxidized in the presence of other oxidizable species (including, aliphatic hydrocarbons, aromatic hydrocarbons, carbon dioxide, hydrogen or carbon monoxide) that may be present in the gas stream. The catalysts and methods therein are particularly applicable to the removal of hydrogen sulfide from gas streams, including natural gas, synthesis gas, and other gas streams generated or used in petroleum refining and processing. This application and its published counterpart are each incorporated by reference herein for teachings and descriptions of selective H2S oxidation catalysts, methods for selective oxidation of H2S; catalytic reactors and catalytic reactor systems.
The present invention is based at least in part on the discovery that mercury in gas streams, such as natural gas and synthesis gas, can be removed or its levels significantly reduced by treatment employing certain selective oxidation processes described in U.S. publication no. 2003 0194366A1.